Various parts and structures are currently designed and built using composites, many of which are bonded together using adhesives. The study of bonded and co-cured composite interface behavior under load is important to the proper design and use of these structures. For example, structural analysis codes are used to predict locations and load levels for failure initiation. Composite manufacturers and manufacturers of structures using composites employ various mechanical testing machines to stress representative composites to failure in order to validate and refine structural performance codes.
Accurately defining the onset of structural failure would be desirable for generating structural analysis codes. However, currently it is not possible to determine the initiation of failure due to the start of a disbond or delamination or cracking from the load-displacement curve produced during mechanical testing (such as bending, tension, compression, and the like). For example, there is no “knee” in the curve that would indicate that a structure has begun to fail. Crack wires and conductive paint can sometimes be used in cases where a delamination initiates at a re-entrant corner, but they cannot be used to detect growth from an internal feature or an existing internal delamination. Many times, test engineers cannot verify where damage is initiating or growing.
In addition, currently a non-destructive inspection (NDI) technician or other operator is required to periodically inspect locations of a structure under test. In such cases, testing has to be stopped (that is, the load is removed from the structure under test) for personnel safety purposes. This is because an operator currently must hold an ultrasonic testing (UT) transducer in place to inspect the unit. However, for personnel safety reasons, an operator can not hold a transducer onto a structure while load is being applied to the structure. Therefore, currently load is removed from the structure, the operator places the transducer on the structure, and the unloaded structure is then inspected. This process can mean that a high-use test machine, possibly with a backlog of testing, may sit idle while waiting for a shift change so the NDI technician can check for cracking or delamination or disbond growth. Moreover, the precision of the dimensional measurement is limited by the damage marking precision (to define the physical extent of the damage) and the technician's ability to discern small changes.
Therefore, improvements may be possible regarding increases in sensitivity, accuracy, and repeatability of measurement data, regarding determining the initiation of failure, and regarding reductions in schedule and labor costs.
The foregoing examples of related art and limitations associated therewith are intended to be illustrative and not exclusive. Other limitations of the related art will become apparent to those of skill in the art upon a reading of the specification and a study of the drawings.